Flux focusing magnetic gear assembly using ferrite magnets or the like

ABSTRACT

The present invention relates generally to a flux focusing magnetic gear assembly using ferrite magnets or the like. The present invention also relates generally to a flux focusing magnetic gear assembly using ferrite magnets or the like that incorporates an outer stator assembly that converts a variable input to a constant output. The present invention further relates generally to an axially aligned flux focusing magnetic gear assembly using ferrite magnets or the like. The improved flux focusing magnetic gear assemblies of the present invention find applicability in traction, wind, and ocean power generation, among other applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application/patent claims the benefit of priority ofU.S. Provisional Patent Application No. 61/542,335, filed on Oct. 3,2011, and entitled “FLUX FOCUSING FERRITE MAGNETIC GEAR,” and U.S.Provisional Patent Application No. 61/673,448, filed on Jul. 19, 2012,and entitled “CONTINUOUSLY VARIABLE MAGNETIC GEAR,” the contents of bothof which are incorporated in full by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to an improved flux focusingmagnetic gear assembly using ferrite magnets or the like. The presentinvention also relates generally to an improved flux focusing magneticgear assembly using ferrite magnets or the like that incorporates anouter stator assembly that converts a variable input to a constantoutput. The present invention further relates generally to an improvedaxially aligned flux focusing magnetic gear assembly using ferritemagnets or the like. The improved flux focusing magnetic gear assembliesof the present invention find applicability in traction, wind, and oceanpower generation, among other applications.

BACKGROUND OF THE INVENTION

Magnetic gear assemblies offer numerous advantages over counterpartmechanical gear assemblies. Magnetic gear assemblies provide acontactless mechanism for speed amplification (i.e. acoustic noises,vibrations, and wear and tear are all reduced), do not requirelubrication (i.e. maintenance costs and pollution are both reduced),have inherent overload protection (i.e. slippage inherently replacesmechanical breakage), and have the potential for high conversionefficiency. Numerous conventional magnetic gear assemblies are known tothose of ordinary skill in the art, typically including a plurality ofpermanent magnets arranged one directly next to another in adjacent orconcentric rings or rotors around one or more axes, with steel poles orthe like interspersed between the adjacent or concentric rings in anintermediate ring or rotor, for example. The result is selectivelyactuated relative rotation of the adjacent or concentric rings orrotors, as well as the intermediate ring or rotor, and speedamplification results. Typically, the flux fields of the magnets arepurposefully magnetized in a radial direction.

For example, referring specifically to FIG. 1, one conventional magneticgear assembly 5 includes an inner rotor 10 including P₁ magnet polepairs that rotates at angular velocity ω₁, a middle rotor 12 includingn₂ ferromagnetic steel poles or the like that rotates at angularvelocity ω₂, and an outer rotor 14 including P₃ magnet pole pairs thatrotates at angular velocity ω₃. The flux fields of the magnets arealigned as illustrated. If the relationship between the poles is chosento be:P ₁ =|P ₃ −n ₂|,  (1)then the inner rotor 10 and the outer rotor 14 interact with the middlerotor 12, via flux linkage, to create space harmonics. The angularvelocities of the rotors are related by:ω₁ =[P ₃/(P ₃ −n ₂)]ω₃ +[n ₂/(n ₂ −P ₃)]ω₂.  (2)If the outer rotor 14 is stationary (i.e. ω₃=0), then:ω₁ =[n ₂/(n ₂ −P ₃)]ω₂ =Gω ₂,  (3)where G is the gear ratio. The above referenced flux linkage, and fluxfocusing, is illustrated specifically in FIG. 2.

Invariably, a rare earth material, such as a neodymium iron boron(Nd—Fe—B) alloy, is used as the permanent magnet material. This canbecome prohibitively expense, and the use of a less expensive ferritematerial is certainly preferred, although the inferior performance ofthe ferrite material must be compensated for via a superior magneticgear assembly design.

As alluded to above, magnetic gear assemblies are ideally suited for usein traction, wind, and ocean power generation applications, amongothers, where, for example, wave energy converters (WECs) or the likeproduce very low speed translational motions (e.g. 0.1-2 m/s) orrotational motions (5-20 rpm). Generally, given such low speeds,extremely large or extremely high force density devices are required togenerate significant power. Exemplary devices include rotary turbogenerators—typically driven by an oscillating airflow, hydraulic motorgenerators—typically driven by a pressurized fluid, and direct drivelinear generators—typically driven by sea motion. It is in conjunctionwith such devices that magnetic gear assemblies prove to be mostvaluable at present, although the potential applications are virtuallylimitless.

Thus, what are still needed in the art are improved low cost magneticgear assemblies using ferrite magnets or the like, that can provideincreased angular velocities and gear ratios, while still providing acontactless mechanism for speed amplification, not requiringlubrication, having inherent overload protection, and having thepotential for high conversion efficiency.

BRIEF SUMMARY OF THE INVENTION

In one exemplary embodiment, the present invention provides a fluxfocusing magnetic gear assembly, comprising: an inner rotor comprising aplurality of concentrically disposed inner magnets separated by aplurality of concentrically disposed inner interstitial members, whereinthe magnetic fields within the plurality of inner magnets are magnetizedazimuthally through their thicknesses such that their opposite poles areat their opposite major planar faces; a middle rotor disposed about theinner rotor and comprising a plurality of concentrically disposed polesseparated by one of a plurality of concentrically disposed gaps andplurality of concentrically disposed middle interstitial members; and anouter rotor disposed about the middle rotor and comprising a pluralityof concentrically disposed outer magnets separated by a plurality ofconcentrically disposed outer interstitial members, wherein the magneticfields within the plurality of outer magnets are magnetized azimuthallythrough their thicknesses such that their opposite poles are at theiropposite major planar faces. The inner interstitial members, the poles,and the outer interstitial members are comprised of a magnetic material,while the middle interstitial members are comprised of air or anonmagnetic material. The inner rotor is disposed about one of a gap anda nonmagnetic shaft. Optionally, the middle rotor comprises theplurality of concentrically disposed poles separated by and interlockedwith the plurality of concentrically disposed middle interstitialmembers. A performance characteristic of the flux focusing magnetic gearassembly is maximized by optimizing a length of each of the plurality ofmagnets and a width of each of the plurality of interstitial members.

In another exemplary embodiment, the present invention provides a fluxfocusing magnetic gear assembly, comprising: an inner rotor comprising aplurality of concentrically disposed inner magnets separated by aplurality of concentrically disposed inner interstitial members, whereinthe magnetic fields within the plurality of inner magnets are magnetizedazimuthally through their thicknesses such that their opposite poles areat their opposite major planar faces; a middle rotor disposed about theinner rotor and comprising a plurality of concentrically disposed polesseparated by one of a plurality of concentrically disposed gaps andplurality of concentrically disposed middle interstitial members; and anouter stator disposed about the middle rotor and comprising one or moreconcentrated or distributed windings. The inner interstitial members andthe poles are comprised of a magnetic material, while the middleinterstitial members are comprised of air or a nonmagnetic material. Theinner rotor is disposed about one of a gap and a nonmagnetic shaft.Optionally, the middle rotor comprises the plurality of concentricallydisposed poles separated by and interlocked with the plurality ofconcentrically disposed middle interstitial members. A performancecharacteristic of the flux focusing magnetic gear assembly is maximizedby optimizing a length of each of the plurality of magnets and a widthof each of the plurality of interstitial members.

In a further exemplary embodiment, the present invention provides anaxial flux focusing magnetic gear assembly, comprising: a high speedrotor comprising a plurality of concentrically disposed high speedmagnets separated by a plurality of concentrically disposed high speedinterstitial members, wherein the magnetic fields within the pluralityof high speed magnets are magnetized azimuthally through theirthicknesses such that their opposite poles are at their opposite majorplanar faces; an intermediate rotor disposed axially adjacent to thehigh speed rotor and comprising a plurality of concentrically disposedpoles separated by one of a plurality of concentrically disposed gapsand plurality of concentrically disposed intermediate interstitialmembers; and a low speed rotor disposed axially adjacent to theintermediate rotor and comprising a plurality of concentrically disposedlow speed magnets separated by a plurality of concentrically disposedlow speed interstitial members, wherein the magnetic fields within theplurality of low speed magnets are magnetized azimuthally through theirthicknesses such that their opposite poles are at their opposite majorplanar faces. The high speed interstitial members, the poles, and thelow speed interstitial members are comprised of a magnetic material,while the intermediate interstitial members are comprised of air or anonmagnetic material. The high speed rotor, the intermediate rotor, andthe low speed rotor are disposed about one of a gap and a nonmagneticshaft and rotate independently. Optionally, the intermediate rotorcomprises the plurality of concentrically disposed poles separated byand interlocked with the plurality of concentrically disposedintermediate interstitial members. A performance characteristic of theaxial flux focusing magnetic gear assembly is maximized by optimizing alength of each of the plurality of magnets and a width of each of theplurality of interstitial members.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated and described herein with referenceto the various drawings, in which like reference numbers are used todenote like assembly components/method steps, as appropriate, and inwhich:

FIG. 1 is a schematic diagram illustrating a conventional magnetic gearassembly;

FIG. 2 is a schematic diagram illustrating a principle of operation ofthe conventional magnetic gear assembly of FIG. 1, as well as, in part,a principle of operation of the magnetic gear assemblies of the presentinvention;

FIG. 3 is a schematic diagram illustrating one exemplary embodiment ofthe flux focusing magnetic gear assembly of the present invention;

FIG. 4 is a perspective view of one possible physical implementation ofthe cage rotor of the flux focusing magnetic gear assembly of FIG. 3;

FIG. 5 is a perspective view of one possible physical implementation ofthe overall flux focusing magnetic gear assembly of FIG. 3;

FIG. 6 is a schematic diagram illustrating another exemplary embodimentof the flux focusing magnetic gear assembly of the present invention,including a middle rotor that utilizes a plurality of keyed andinterlocked poles and interstitial members;

FIG. 7 is a schematic diagram illustrating a further exemplaryembodiment of the flux focusing magnetic gear assembly of the presentinvention, including a stator that replaces the outer rotor thereof; and

FIG. 8 is a schematic diagram illustrating a still further exemplaryembodiment of the flux focusing magnetic gear assembly of the presentinvention, utilizing an axial rotor alignment.

DETAILED DESCRIPTION OF THE INVENTION

Referring specifically to FIG. 3, in one exemplary embodiment, the fluxfocusing magnetic gear assembly 15 of the present invention includes aninner rotor 20 including P₁ magnet pole pairs that rotates at angularvelocity ω₁, a middle rotor 22 including n₂ ferromagnetic steel poles orthe like that rotates at angular velocity ω₂, and an outer rotor 24including P₃ magnet pole pairs that rotates at angular velocity ω₃. Themagnetization directions of the magnets 26 are aligned as illustrated,in a flux focusing arrangement, also referred to herein as a spoke typearrangement. Specifically, the magnets 26 are purposefully magnetized ina substantially azimuthal direction, contrary to the radial directionthat has been used conventionally.

Related to the inner rotor 20, the plurality of magnets 26 are separatedby a plurality of rectangular, wedge shaped, or annular steel teeth 28or the like for enhancing flux focusing functionality. The performanceof the magnetic gear assembly 15 is, in part, optimized by adjusting thelength of the magnets 26, L₁, relative to the available angular span,θ_(s1), provided by each of the steel teeth 28 or the like. Similarly,related to the outer rotor 24, the plurality of magnets 26 are separatedby a plurality of rectangular, wedge shaped, or annular steel teeth 28or the like for enhancing flux focusing functionality. Again, theperformance of the magnetic gear assembly 15 is, in part, optimized byadjusting the length of the magnets 26, L₃, relative to the availableangular span, θ_(s3), provided by each of the steel teeth 28 or thelike. The middle rotor 22 includes a plurality of steel poles 32 or thelike, separated by air gaps or the like, in this exemplary embodiment.It should be noted that the inner rotor 20, the middle rotor 22, and theouter rotor 24 are disposed about a common central axis 40 and areseparated by small air gaps 42 concentrically, such that they may freelyrotate with respect to one another in a frictionless manner. As isdescribed in greater detail herein below, a large number ofcharacteristics and parameters can be, and are, optimized for enhancedperformance.

In order to take advantage of flux focusing, the inner rotor 20 shouldhave more than 4 poles. For example, the flux focusing magnetic gearassembly 15 can have P₁=4 pole pairs, n₂=17 steel poles, P₃=13 polepairs on the outer rotor 24. If the outer rotor 24 is stationary, thenω₃=0, and the gear ratio is:ω₁ =[n ₂/(n ₂ −P ₃)]ω₂ =Gω ₂,  (4)where G=4.25ω₂. This combination of poles was chosen for illustrationpurposes because it has a low cogging factor, C_(f)=1. The coggingfactor is defined by:C _(f)=(2P ₂ n ₂)/[LCM(2P ₁ ,n ₂)],  (5)where LCM is the lowest common multiple.

Referring specifically to FIG. 4, in one exemplary embodiment, themiddle rotor 22, or cage rotor, includes a plurality of end plates,including a high speed retaining plate 50 and a low speed retainingplate 52, between which the plurality of steel poles 32 or the like aredisposed, and to which the inner rotor 20, or high speed rotor, andouter rotor 24, or low speed rotor, are magnetically coupled.

Referring specifically to FIG. 5, in one exemplary embodiment, theoverall construction of the flux focusing magnetic gear assembly 15 isillustrated, including the inner rotor 20, the middle rotor 22, theouter rotor 24, the various magnets 26, the various steel teeth 28 orthe like, and the various steel poles 32 or the like. It will be readilyapparent to those of ordinary skill in the art that slight modificationscan be made to this bulk configuration without changing thefunctionality thereof.

Exemplary specifications are provided in Table 1 below, for the purposeof providing relative characteristics and dimensions only.

TABLE 1 Exemplary Preliminary Flux Focusing Magnetic Gear AssemblySpecifications Description Value Units Inner rotor Pole pairs, p₁ 4Inner radius, r_(i1) 13 mm Outer radius, r_(o1) 33 mm Steel pole span,θ_(s1) π/8  rad. Airgap, g 0.5 mm Cage rotor Steel poles, n₂ 17 — Innerradius, r_(i1) 33.5 mm Outer radius, r_(o1) 46.5 mm Steel pole span,θ_(s2) π/15 rad. Outer cylinder Pole pairs, p₃ 13 — Inner radius, r_(i3)47 mm Outer radius, r_(o3) 59 mm Steel pole span, θ_(s3) π/26 rad.Airgap, g 0.5 mm Material Magnet, Hitachi 0.46 T NMF12F Steelresistivity 5.1 × 10⁻⁷ Ω-m Stack length, d 152.4 mm

Flux focusing is achieved by first changing the area with which the fluxflows through the width of the steel pole 28 relative to the length ofthe magnets 26. The relation between air gap flux density and magnetflux density is given by:B _(g) W _(s) d=B _(m)2L ₁ d,  (6)where B_(g) is the air gap flux density, B_(m), is the magnet fluxdensity, L₁=r_(o1)−r_(i1), d is the active stack length, andW_(s)=r_(o1)θ_(s1) is the angular span of the inner rotor steel poles28. The flux concentration ratio is then given by:C _(φ1) =B _(g) /B _(m)=(2/θ_(s1))[1−(r _(i1) /r _(o1))].  (7)C_(φ1)=3.06 is obtained for the inner rotor 20 in the present example.

The flux concentration ratio, C_(φ3), can also be varied to determinethe optimum length for the outer rotor magnets 26, using:C _(φ3) =B _(g3) /B _(m3)=(2/θ_(s3))[(r _(o3) /r ⁻³)−1].  (9)C_(φ3)=6.77, which corresponds to L₃=15 mm, gives the highest torquedensity, as an example.

The cage rotor steel pole length, L₂, can further be varied, from 3 to24 mm in the present example, by way of illustration.L ₂ r _(o2) −r _(i2).  (10)It is observed that L₂=6 mm provides the highest torque density andlowest torque ripple.

The cage rotor steel pole span, θ_(s2), can still further be varied,keeping other parameters constant.W _(s23)=θ_(s2)/θ_(s3).  (11)θ_(s2)=14 degrees and W_(s23)=2.36 provides the maximum torque density,by way of illustration.

As a result, a final flux focusing magnetic gear assembly design isachieved, after parametric optimization (which is example specific),including the design parameters provided in Table 2, for the purpose ofproviding relative characteristics and dimensions only.

TABLE 2 Exemplary Final Flux Focusing Magnetic Gear AssemblySpecifications Description Value Units Inner rotor Pole pairs, p₁ 4Inner radius, r_(i1) 13 mm Outer radius, r_(o1) 33 mm Steel pole span,θ_(s1) π/8  rad. Airgap, g 0.5 mm Cage rotor Steel poles, n₂ 17 — Innerradius, r_(i1) 33.5 mm Outer radius, r_(o1) 39.5 mm Steel pole span,θ_(s2) 7π/90 rad. Outer cylinder Pole pairs, p₃ 13 — Inner radius,r_(i3) 40 mm Outer radius, r_(o3) 15 mm Steel pole span, θ_(s3) π/26rad. Airgap, g 0.5 mm Material magnet, Hitachi 0.46 T NMF12F Steelresistivity 5.1 × 10⁻⁷ Ωm Stack length, d 152.4 mm

It should be noted that the common central shaft portion of the fluxfocusing magnetic gear assembly 15 of the present invention can be open,nonmagnetic, or complex, as is illustrated in FIG. 4, for example, withthe middle rotor 22, or cage rotor, including a plurality of end plates,including a high speed retaining plate 50 and a low speed retainingplate 52, between which the plurality of steel poles 32 or the like aredisposed. An input shaft (not illustrated) and bearing housing (notillustrated) can then be included on the high speed retaining plate 50and low speed retaining plate 52, respectively.

Referring specifically to FIG. 6, in another exemplary embodiment, itshould also be noted that the middle rotor 22 can include a plurality ofkeyed and interlocked poles 60 and interstitial members 62 (a pluralityof slotted and recessed poles 60 and interstitial members areillustrated, as an example). The poles 60 include a soft magneticcomposite (SMC) material, such as bound steel particles or the like, andthe interstitial members 62 include a nonmagnetic material, such asbundled carbon fibers or the like, creating a structurally unifiedmiddle rotor 22. Such an arrangement may also be applied to otherstages, as desired.

Referring specifically to FIG. 7, in a further exemplary embodiment, theouter rotor 24 (see FIG. 3) of the flux focusing magnetic gear assembly75 is replaced with a stator 70, including a stationary winding that isused to create a rotating field, rather than a fixed field, when usingpermanent magnets 26. The stator electrical frequency, ω_(c), andmechanical frequency, ω₃, are related by ω₃=ω_(c)/P₃. Equation (1) isthen:ω₁=[1/(P ₃ −n ₂)]ω_(c) +[n ₂/(n ₂ −P ₃)]ω₂.  (12)Therefore, the use of windings results in the gear ratio beingcontinuously variable. With P₁=4, P₃=13, and n₂=17, the speedrelationship is:ω₁=−0.25ω_(c)+4.25ω₂.  (13)

It is then noted that, if the input speed, ω₂, from a turbine, forexample, is varying, then the output mechanical speed, ω₁, can be madeconstant by controlling the frequency, ω_(c). At the same time, themechanical speed is amplified. The windings 75 shown are concentratedwindings, however, distributed windings or the like can also be used. Asurface mounted rotor, rather than a spoke type rotor, can also be usedin this embodiment.

The torque magnitude, and therefore the power flow, can also be variedby varying the converter voltage level. This topology was studied byothers for a traction motor. However, in this analysis, only the highspeed rotor was rotating. A continuously variable magnetic gear (CVMG)with two rotors has not been studied by others. By combining this CVMGwith a low cost permanent magnet synchronous generator (PMSG) or thelike, the resultant system can act like a gearbox and doubly fedinduction generator (DFIG), but without the need for brushes or amechanical gearbox. Also, unlike a direct drive system, the PMSG can besized to be relatively small because the input speed into the generatoris high. In order to further increase the rotational speeds up to anacceptable level for the generator, a second and possibly third magneticgear set can be used (wind turbines typically use multiple gearboxes,for example). This topology is particularly low cost because it requiresminimal energy storage.

A further possibility that is available when using a magnetic gear withwindings is the ability to create high speed unidirectional rotationalmotion from low speed oscillatory motion. This is an importantcharacteristic for WECs, since the speed of the prime mover is typicallyoscillating. If the stator 70 is replaced by a dual winding and, as anexample, winding one is designed to create P₃=13 pole pairs, whilewinding two creates P₃=21 pole pairs, then it is noted by looking atequation 12 that, if only the winding one is turned on, since n₂=17, thespeed relationship is ω₁=−0.25ω_(c)+4.25ω₂, while if only the windingtwo is turned on, the speed relationship is, ω₁=+0.25ω_(c)−4.25ω₂.Therefore, by choosing to turn on the correct stator winding, anoscillatory WEC rotation, ω₂, can be converted to speed amplifiedunidirectional rotation by a noncontact means; the speed smoothing canbe achieved by added or subtracting the electrical frequency, ω_(c).

Referring specifically to FIG. 8, in a still further exemplaryembodiment, the flux focusing magnetic gear assembly 85 of the presentinvention includes a high speed rotor 80 including P₁ magnet pole pairsthat rotates at angular velocity ω₁, an intermediate rotor 82 includingn₂ ferromagnetic steel poles or the like that rotates at angularvelocity ω₂, and a low speed rotor 84 including P₃ magnet pole pairsthat rotates at angular velocity ω₃. The flux fields of the magnets 26are aligned as previously illustrated, in a flux focusing and fluxconcentrating arrangement, also referred to herein as a spoke typearrangement. Specifically, the flux fields of the magnets 26 arepurposefully magnetized in a substantially azimuthal direction, contraryto the radial direction that has been used conventionally.

Related to the high speed rotor 80, the plurality of magnets 26 areseparated by a plurality of rectangular, wedge shaped, or annular steelteeth 28 or the like for enhancing flux focusing functionality. Theperformance of the magnetic gear assembly 85 is, in part, optimized byadjusting the depth of the magnets 26, l_(m1), relative to the availableangular span, θ_(1s), provided by each of the steel teeth 28 or thelike. Similarly, related to the low speed rotor 84, the plurality ofmagnets 26 are separated by a plurality of rectangular, wedge shaped, orannular steel teeth 28 or the like for enhancing flux focusingfunctionality. Again, the performance of the magnetic gear assembly 85is, in part, optimized by adjusting the depth of the magnets 26, l_(m3),relative to the available angular span, θ_(3c), provided by each of thesteel teeth 28 or the like. The intermediate rotor 82 includes aplurality of steel poles 28 or the like, separated by air gaps or thelike, in this exemplary embodiment. The high speed rotor 80, theintermediate rotor 82, and the low speed rotor 84 are disposedsubstantially adjacent to one another about a common central axis andare separated by small air gaps axially, such that they may freelyrotate with respect to one another in a frictionless manner. Again, alarge number of characteristics and parameters can be, and are,optimized for enhanced performance. Exemplary configurations areprovided in Table 3 below, for the purpose of providing relativecharacteristics and dimensions only.

TABLE 3 Exemplary Axial Flux Focusing Magnetic Gear AssemblySpecifications ¤ Description ¤ Value Units ¤ ¤ ¶ ¤ Description ¤ ValueUnits ¤ ¤ High Pole pairs, p₁ ¤ 7 ¤ — ¤ ¤ High Pole pairs, p₁ ¤ 6 ¤ — ¤¤ Speed Stack length, 100 ¤ mm ¤ ¤ ¶ Speed Stack length, 60 ¤ mm ¤ ¤Rotor ¤ l_(m1) (Z-axis) ¤ Rotor ¤ l_(m1) (Z-axis) ¤ Width, θ_(1s)¤2π/28 ¤ rad. ¤ ¤ Width, θ_(1s) ¤ 2π/24 ¤ rad. ¤ ¤ Airgap (axial)¤ 0.5 ¤mm ¤ ¤ Airgap (axial) ¤ 0.5 ¤ mm ¤ ¤ Low Pole pairs, p₃ ¤ 15 ¤ — ¤ ¤ ¶Low Pole pairs, p₃ ¤ 19 ¤ — ¤ ¤ speed Stack length, 30 ¤ mm ¤ ¤ speedStack length, 40 ¤ mm ¤ ¤ rotor¶ l_(m3) (Z-axis) ¤ rotor¶ l_(m3)(Z-axis) ¤ ¤ Width θ_(3s)¤ 2π/60 ¤ rad. ¤ ¤ ¤ Width θ_(3s)¤ 2π/76 ¤rad. ¤ ¤ Airgap (axial)¤ 0.5 ¤ mm ¤ ¤ ¶ Airgap (axial) ¤ 0.5 ¤ mm ¤ ¤Steel Steel poles, n₂¤ 22 ¤ — ¤ ¤ Steel Steel poles, n₂ ¤ 25 ¤ — ¤ ¤poles¶ Stack length, 7 ¤ mm ¤ ¤ ¶ poles¶ Stack length, 7 ¤ mm ¤ ¤ ¤l_(s2) (Z-axis) ¤ ¤ l_(s2) (Z-axis) ¤ Width θ_(2s)¤ 12 ¤ ¤ Widthθ_(2s) ¤ 9.4 ¤ ¤ ¤ Airgap (axial)¤ 0.5 ¤ mm ¤ ¤ Airgap (axial) ¤ 0.5 ¤mm ¤ ¤ Material Magnets: Hitachi 0.46 ¤ T¤ ¤ ¶ Material Magnets: Hitachi0.46 ¤ T ¤ ¤ NMF12F ¤ NMF12F ¤ Steel resistivity, 0 ¤ Ωm ¤ ¤ ¶ Steelresistivity,¶ 0 ¤ Ωm ¤ Grade 416 ¤ Grade 416 ¤ ¤ Total axial 138 ¤ mm ¤¤ ¤ Total axial 108 ¤ mm ¤ ¤ length, ¤ length, ¤ Inner radius, 140 ¤mm ¤ ¤ ¶ Inner radius, 70 ¤ mm ¤ ¤ τ_(i1) ¤ τ_(i1) ¤ Outer radius, 250 ¤mm ¤ ¤ Outer radius, 185 ¤ mm ¤ ¤ τ_(o1) ¤ τ_(o1) ¤

Although the present invention has been illustrated and described hereinwith reference to preferred embodiments and specific examples thereof,it will be readily apparent to those of ordinary skill in the art thatother embodiments and examples may perform similar functions and/achievelike results. All such equivalent embodiments and examples are withinthe spirit and scope of the present invention, are contemplated thereby,and are intended to be covered by the following claims.

What is claimed is:
 1. A flux focusing magnetic gear assembly,comprising: an inner rotor comprising a plurality of concentricallydisposed inner magnets separated by a plurality of concentricallydisposed inner interstitial members, wherein the magnetic fields withinthe plurality of inner magnets are magnetized azimuthally through theirthicknesses such that their opposite poles are at their opposite majorplanar faces; a middle rotor disposed about the inner rotor andcomprising a plurality of concentrically disposed poles separated by oneof a plurality of concentrically disposed gaps and plurality ofconcentrically disposed middle interstitial members; and an outer rotordisposed about the middle rotor and comprising a plurality ofconcentrically disposed outer magnets separated by a plurality ofconcentrically disposed outer interstitial members, wherein the magneticfields within the plurality of outer magnets are magnetized azimuthallythrough their thicknesses such that their opposite poles are at theiropposite major planar faces.
 2. The flux focusing magnetic gear assemblyof claim 1, wherein the inner interstitial members, the poles, and theouter interstitial members are comprised of a magnetic material and themiddle interstitial members are comprised of one of air and anonmagnetic material.
 3. The flux focusing magnetic gear assembly ofclaim 1, wherein the inner rotor is disposed about one of a gap and anonmagnetic shaft.
 4. The flux focusing magnetic gear assembly of claim1, wherein a performance characteristic of the flux focusing magneticgear assembly is maximized by optimizing a length of each of theplurality of magnets and a width of each of the plurality ofinterstitial members.